The present invention relates to an elastomer obtained by radical polymerization of a liquid composition of low viscosity based on a hydroxylated diene oligomer.
Liquid compositions of low viscosity based on polydiene with hydroxylated endings, on polymerization initiators and on reactive diluents comprising 2 or 3 (meth) acrylate groups per molecule and having a molar mass of 250 to 1000 g/mol are already known from JP-A-06 28 72 49. These compositions are thermally crosslinked to give polymers used as coating or adhesives. However, these polymers have the disadvantage of having brittle behaviour, of not being very elastomeric and of having very low resistances to folding and very low values of elongation at break.
The aim of the present invention is to obtain a liquid composition of low viscosity based on a hydroxylated diene oligomer having good stability on storage. This composition must in particular be able to be used, without employing isocyanate, as coating for a metal surface or plastic surface. It must result, by thermal or photochemical radical polymerization, in an elastomer which does not exhibit the disadvantages reported above.
This aim is achieved by preparing first of all a hydroxylated prepolymer obtained by condensation of a diene oligomer comprising hydroxylated endings, optionally mixed with a chain extender, in particular a diol, with a polyisocyanate compound in a ratio by weight such that the ratio of the molar concentration of the isocyanate groups (xe2x80x94Nxe2x95x90Cxe2x95x90O) to the molar concentration of the hydroxyl groups (xe2x80x94OH) is from 0.05 to the value q; q being chosen so as not to achieve gelling of the prepolymer; this ratio being determined while taking into account the possible presence of the chain extender.
The prepolymer obtained comprises an unsaturated backbone of polydiene type, urethane groups connecting several carbonaceous backbones of the starting molecules and of hydroxylated diene oligomers. This prepolymer is randomly functionalized at the chain end by hydroxyl groups, because the polyisocyanate is reacted in amounts such that the isocyanate functional groups (xe2x80x94Nxe2x95x90Cxe2x95x90O) are in deficiency with respect to the hydroxyl groups present.
A composition based on this prepolymer which is stable on storage and which is capable of giving an elastomeric polymer by thermal or photochemical radical crosslinking is characterized by the content of the following constituents:
100 parts by weight of the urethane prepolymer obtained by condensation of a hydroxylated diene oligomer, optionally mixed with a chain extender, with a polyisocyanate compound in a ratio by weight such that the ratio of the molar concentration of the isocyanate groups (xe2x80x94Nxe2x95x90Cxe2x95x90O) to the molar concentration of the hydroxyl groups (xe2x80x94OH) is from 0.05 to the value q, this value q being chosen so as not to achieve gelling of the prepolymer,
from 1 to 200 parts by weight of acrylate oligomers,
from 1 to 100 parts by weight of acrylate monomers,
from 0 to 100 parts by weight of plasticizers,
from 0 to 10 parts by weight of additives,
from 0.1 to 5 parts by weight of radical initiator.
This composition, which is stable under the usual storage conditions (from room temperature to 60xc2x0 C., for example), has the advantage of resulting, after thermal or photochemical crosslinking, in an elastomeric polymer without the crosslinking proper requiring the addition of a polyisocyanate.
Thus, the final user of the composition will not himself have to use a polyisocyanate compound, which exhibits the advantage of not handling compounds of this type known for their toxicity.
The composition is preferably characterized in that the content of the constituents is:
100 parts by weight of the said urethane prepolymer,
from 50 to 100 parts by weight of acrylate oligomer(s),
from 10 to 50 parts by weight of acrylate monomer(s),
from 0 to 100 parts by weight of plasticizers,
from 0 to 5 parts by weight of additives,
from 0.1 to 5 parts by weight of radical initiator.
The gelling of the prepolymer is defined by the loss of its ability to flow.
The above constituents are defined in the following way:
a) the diene oligomers comprising hydroxylated endings, also known as polydienepolyols, which can be used according to the present invention comprise hydroxytelechelic conjugated diene oligomers which can be obtained by various processes.
Mention may be made, among these processes, of the radical polymerization of conjugated dienes having from 4 to 20 carbon atoms in the presence of hydrogen peroxide or optionally of an azo compound, such as 2,2xe2x80x2-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], or alternatively the anionic polymerization of the conjugated diene having from 4 to 20 carbon atoms in the presence of formaldehyde or of ethylene oxide and of a basic catalyst, such as naphthalenelithium or sec-butyllithium.
According to the present invention, the conjugated diene for obtaining the polydienepolyol is chosen from the group comprising butadiene, isoprene, chloroprene, 1,3-pentadiene and cyclopentadiene. The number-average molar mass (Mn) of the polydienepolyols which can be used can be from 500 to 15,000 g/mol and is preferably from 1000 to 5000 g/mol.
Use is advantageously made of a polydiene based on butadiene. The polydienepolyol preferably has the following characteristics:
It comprises from 70 to 85 mol %, preferably 80 mol %, of "Parenopenst"CH2xe2x80x94CHxe2x95x90CHxe2x80x94CH2"Parenclosest" units and from 15 to 30 mol %, preferably 20 mol %, of 
units.
Among polydienepolyols, hydroxylated copolymers of conjugated dienes and of vinyl and acrylic monomers, such as, for example, styrene and/or acrylonitrile, are also suitable.
It would not be departing from the invention if use were made of hydroxytelechelic butadiene oligomers epoxidized on the chain.
The polydienepolyols have a hydroxyl number, expressed in meq/g, of between 0.5 and 5 and their viscosity is between 1000 and 100,000 mPaxc2x7s at 30xc2x0 C.
Mention may be made, as illustration of polydienepolyols which can be used in the present invention, of the polybutadienes comprising hydroxylated endings sold by the company Elf Atochem S.A. under the names Poly Bd(copyright) R-45HT and Poly Bd(copyright) R-20LM.
According to the present invention, the polyisocyanate compound used can be an aromatic, aliphatic or cycloaliphatic compound having at least two isocyanate functional groups in its molecule.
Mention may be made, as illustration of aromatic diisocyanate, of diphenylmethane 4,4xe2x80x2-diisocyanate (MDI), liquid modified MDIs, polymeric MDIs, toluene 2,4- and 2,6-diisocyanate (TDI) and their mixture, xylene diisocyanate (XDI), tetramethylxylene diisocyanate (TMXDI), para-phenylene diisocyanate (PPDI) or naphthalene diisocyanate (NDI).
Among aromatic diisocyanates, the invention preferably relates to diphenylmethane 4,4xe2x80x2-diisocyanate. In this specific case, the NCO/OH molar ratio is advantageously from 0.05 to 0.60.
Mention may be made, as illustration of aliphatic polyisocyanate, of hexamethylene diisocyanate (HDI) and its derivatives (isocyanate, biruet) or trimethylhexamethylene diisocyanate (HMDI).
Mention may be made, as illustration of cycloaliphatic polyisocyanate, of isophorone diisocyanate (IPDI) and its derivatives, dicyclohexylmethane 4,4xe2x80x2-diisocyanate (H12MDI) and cyclohexyl diisocyanate (CHDI).
According to the invention, the chain extender can be chosen from diols. Their molecular mass can be between 62 and 500 g/mol.
Mention will be made, as illustration of such compounds, of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, 2-ethyl-1,3-hexanediol, N,N-bis(2-hydroxypropyl)aniline, 3-methyl-1,5-pentanediol and the mixture of at least two of the abovementioned compounds.
The diol or the mixture of at least two of these diols can advantageously be present at up to 50 parts by weight.
Polyamines can also be used as chain extenders. Their molecular mass can be between 60 and 500 g/mol.
Mention will be made, as illustration of such polyamines, of ethylenediamine, diphenylmethanediamine, isophoronediamine, hexamethylenediamine or diethyltoluenediamine.
The polyacrylate oligomers are advantageously chosen with number-average molar masses Mn ranging from 100 to 5000 g/mol. These oligomers are, for example, urethane acrylates or epoxy acrylates.
The (meth)acrylate monomers are advantageously chosen from mono-, di-, tri- or tetrafunctional alkyl acrylates and methacrylates preferably having a boiling point greater than 150xc2x0 C. at normal pressure, if thermal radical initiation is envisaged. The choice is preferably made from the monofunctional (meth)acrylates, isobornyl acrylate, isobornyl methacrylate, dicyclopentenyloxyethyl methacrylate, methoxyethyl acrylate, ethoxyethyl methacrylate, isooctyl acrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, nonyl acrylate or lauryl methacrylate.
The plasticizers are advantageously chosen from dialkyl and alkyl benzyl phthalates, alkyl sebacates or alkyl adipates.
The additives can be conventional additives known to the person skilled in the art, such as antioxidants, UV stabilizers or radical stabilizers. The formulation can also optionally be charged with inorganic fillers known as such by a person skilled in the art. Mention may be made, as examples, of calcium carbonates, silica or kaolin. Pigments can also be added to the formulation.
The radical initiators can be conventional peroxides, the decomposition of-which and therefore the production of free radicals from which is provided by the thermal route, for example di-tert-butyl peroxide, dicumyl peroxide, tert-butyl perbenzoate or 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, or two-component mixtures of peroxides and of coinitiators (for example, tertiary amines or cobalt salts), which produce free radicals at ambient temperature, or photoinitiator salts capable of generating free radicals under irradiation with ultraviolet radiation or by electron bombardment (Electron Beam (EB)).
Depending on the type of initiator used, the formulations according to the invention will be of the single-component (thermal or photochemical) or two-component (chemical) type.
The invention also relates to a process for the preparation of the composition, characterized in that the diene oligomer comprising hydroxyl endings, optionally mixed with the chain extender, is condensed with the polyisocyanate compound and then the other constituents are subsequently added in order to obtain the said liquid composition of low viscosity.